Rotating stirring device

Abstract

 Rotating stirring device, particularly for mixing heterogeneous systems in mechanical stirred reactors, fit to be joined to a drive shaft coaxial with respect to the device rotation axis, which comprises a plurality of extended and substantially C-shaped stirring elements, symmeterically arranged with respect to the rotation axis and joined each other at the respective ends correspondently with the rotation axis. Such a device permits to obtain a very efficient material exchange among different phases, substantially remaining constant with the increase of the volume of the mass put under stirring, and it is also characterized by high mixing homogeneity and particularly low shear stress values.

Description

[0001] The present invention relates to a rotating stirring device, particularly for mixing heterogeneous systems in mechanical stirred reactors.

[0002] Various kinds of mechanical stirred reactors are known in the art, generally of cylindric shape, in which processes are carried out where a continuous and uniform mixing of various components and an efficient mass exchange between different phases (for instance, gas-liquid, gas-liquid-solid, liquid-solid, liquid-liquid, etc.) are required. The mechanical stirring is commonly carried out through a stirrer put inside the reactor in axial position. The particular stirrer shape (turbine, blade, propeller, etc.) is mainly chosen according to the viscosity of the mass to be put under stirring (to this purpose see "Advances in Chemical Engineering", vol. 17, pag. 5-8, Academic Press, 1992).

[0003] A common limit to all the stirring devices of the art is represented, in the case of gas-liquid systems, by a generally not high mass transfer coefficient between the phases and which considerably decreases when an increase with time of the volume of the mass put under stirring occurs. This is the case, for instance, of polymerization reactions in emulsion or in suspension in which at least one of the monomers is present in the gas phase: during the polymerization a gradual increase of the volume and of the solid content of the liquid phase occurs. A lowering of the mass transfer between the gas phase and liquid phase and consequently a reduction of the process yield derives therefrom.

[0004] The difficulty to obtain satisfactory mass exchanges between the various phases imposes moreover some limits in the reactor shape. In particular, with the traditional stirring systems the use of reactors with a high height/diameter ratio, which would assure a more efficient thermal exchange, is in fact impossible.

[0005] In the gas-liquid systems, in order to improve the mass exchange between the phases, the use of devices distributing the gas inside the liquid phase (spargers) is known. They are in practice toroidal shaped devices, endowed with holes, through which the gas is bubbled. Such devices require however a very frequent maintenance, as they tend to be easily clogged, especially in the case in which in the liquid phase a solid phase is present or is formed during the reaction (it is the case, for instance, of the polymerization reactions).

[0006] The Applicant has now found a new type of rotating stirring device as described hereinunder, particularly suited for mixing heterogeneous systems, which permits to obtain a very efficient mass exchange between different phases and which remains substantially constant when the volume of the mass under stirring increases. Moreover a high mixing homogeneity, with particularly low shear stress values, is achieved.

[0007] It is therefore object of the present invention a rotating stirring device, particularly for mixing heterogeneous systems inside mechanical stirred reactors, fit to be joined to a drive shaft coaxial with respect to the device rotation axis, characterized in that it comprises a plurality of extended and substantially C-shaped stirring elements, said stirring elements being symmetrically arranged with respect to the rotation axis and being joined each other at the respective ends in correspondence with the rotation axis.

[0008] The characteristics of the invention will be now better illustrated by the following description and by the enclosed drawings relating to a non-limitative embodiment, in which the figures show:

Fig. 1, a longitudinal section along the plan X-X';

Fig. 2, a top view.

[0009] The stirring device (1) shown in the above figures comprises three stirring elements (2), which, according to a preferred embodiment of the present invention, are concave blade-shaped with the concavity towards the rotation direction of the device itself. The stirring elements (2) are symmetrically arranged with respect to the rotation axis, each forming with respect to the other an angle (A) of 120°.
Each stirring element (2) consists of a lower end part (3), a vertical part (4) and an upper end part (5). As shown in Fig. 1, the stirring device (1) preferably shows a tapering towards the upper part, wherefore the lower end part (3) is longer than the upper end part (5), so that the vertical part (4) defines with the rotation axis an angle (B) comprised between 0° to 10°. Such tapering has the function of obtaining a more homogeneous distribution of the axial displacement inside the reactor.

[0010] The three stirring elements (2) are joined each other through the lower (6) and upper (7) connection elements, on which the lower end (3) and upper end (5) parts are connected respectively. The upper connection element (7) is solidly joined to a drive axis, fit to give the rotation to the stirring device (1).

[0011] The lower edge of the lower end part (3) forms with the horizontal plane an angle (C) comprised between 0° and 45°, according to the particular shape of the reactor bottom, which is usually concave.

[0012] As to the size characteristics of the stirring device object of the present invention, these ones essentially depend from the reactor sizes in which the device has to be employed.

[0013] Considering a substantially cylinder-shaped reactor having a diameter (T) and a height (V), the sizes of the stirring device object of the present invention are preferably the following:

• base diameter (D) of the stirring device (1): from 1/3 to 2/3 of the diameter (T);
• bending radius (E) of the stirring elements (2): higher than or equal to 1/3 of the diameter (T);
• height (F) of the lower end part (3): higher than or equal to 1/5 of the diameter (D);
• Width (G) of the vertical part (4): from 1/20 to 1/4 of the diameter (T);
• height (L) of the stirring elements (2): from 3/2 of the diameter (D) to a value about equal to the height (V):

   Of course the maximum value of the height (L) depends from the particular shape of the reactor and it must however be such as to allow the free rotation of the device inside the reactor.

[0014] As regards the height (M) of the upper end part (5), this is generally uninfluential on the stirring effectiveness, as it remains, under usual conditions, outside the mass put under stirring. However, the height (M) is generally at least equal to 1/20 of the diameter (T).

[0015] The values given above are merely indicative and can be modified according to the characteristics of the particular heterogeneous system considered, such as, viscosity, density, solid contents, number of phases, etc.

[0016] It is also evident that to the embodiment previously illustrated, various changes, adjustments, variants and replacements of elements with other functionally equivalent elements can be carried out, without being however out of the scope of the claims reported hereinafter.

[0017] Thanks to the high mass exchange between the different phases and to the low shear stress, the present invention is particularly advantageous for reactors to be used in:

• (co)polymerization reactions in emulsion, in particular of fluorinated olefinic monomers, for the production of (co) polymers, such as, for example: homopolymers of tetrafluoroethylene and its elastomeric or plastomeric copolymers with hexafluoropropene, fluorovinylethers, such as perfluoropropylvinylether or perfluoromethylvinylether; homopolymers of vinylidene fluoride and its elastomeric or plastomeric copolymers with hexafluoropropene, tetrafluoroethylene, fluorovinylethers, fully hydrogenated olefins, brominated and/or iodinated vinyl comonomers, etc.;
• (co)polymerization reactions in suspension, in particular of fluorinated olefinic monomers, for the production of (co)polymers such as, for instance, ethylene/tetrafluoroethylene (ETFE) or ethylene/chlorotrifluoroethylene (CTFE) copolymers;
• fermentation reactions for the production of active principles for pharmaceutical use, in particular those reactions in which shear stress sensitive microorganisms and/or products are present;
• processes for slurry preparation, in particular those processes in which shear stress sensitive products are employed (for instance preparation of zeolites dispersions).

[0018] As previously already pointed out, the stirring device of the present invention assures a reaction rate quite constant in time, with homogeneous mixing and low shear stress. In particular, in the case of polymerization reactions in emulsion, a low shear stress is extremely advantageous, as it allows to obtain stable polymerization latexes and it avoids the undesired formation of polymer coagula. Such coagula, as well known, besides fouling the reactor with subsequent maintenance problems, cause various inconveniences, such as reduction of the thermal exchange coefficient, contamination of the polymer, lowering of the reaction rate, etc.

[0019] In case of polymerizations in suspension, the stirring device of the present invention permits moreover to obtain an homogeneous mixing also for contents of suspended solid higher than 50% by weight.

[0020] The particular effectiveness of the present invention in the mixing of heterogeneous systems is evident from the data reported hereinbelow, which compare the stirring device of the present invention (Fig. 3) with a device of the prior art (Fig. 4), in which the stirring is obtained through two Rushton turbines placed on the same rotation axis.

[0021] Fig. 3 shows a section of the stirring device object of the present invention placed into a reactor (8) provided with a baffles couple (9) symmetrically placed inside the reactor (8). The level of the liquid put under stirring is indicated by H. The real sizes of the various elements of the system reactor + stirrer employed in the measurements are the following:
   Reactor (notation of Fig. 3):
   T = 202 mm; a = 150 mm; b = 36 mm; c = 7 mm; d = 6 mm; e = 60 mm; f = 110 mm; g = 324 mm;
   Stirrer (notation of Figs. 1 and 2):
   A = 120°; B = 2.42°; C = 10°; D = 112 mm; E = 60 mm; F = 54 mm; G = 14 mm; L = 420 mm; M = 20 mm.

[0022] Fig. 4 shows a section of the device of the prior art (10), consisting of a couple of six blades (12) Rushton turbines (11), being said turbines (11) set on a rotation shaft (13), placed into a reactor (14) provided with four baffles (15) symmetrically placed inside the reactor (14). The level of the liquid put under stirring is indicated by H. The real sizes of the various elements of the system reactor + stirrer employed in the measurements are the following:
   T = 202 mm; m = 20.2 mm; n = 100 mm; p = 25 mm; q = 20 mm; r = 67.3 mm; s = 200 mm.

[0023] For each system stirrer + reactor the mass transfer coefficient (k₁) of the oxygen between the liquid phase (water) and gas phase (air) has been measured, according to the method described by Y. Imai, H. Takei and M. Matsumura in "Biotechnology and Bioengineering", Vol. XXIX, p. 982-993 (1987). The measurements have been carried out at different filling levels of the reactor.

[0024] The results are reported in Fig. 5. In abscissa the ratio H/T is reported, that is, the ratio between liquid height (H) and reactor diameter (T), in ordinates the product

, where k₁ is the mass transfer coefficient (expressed in m/sec), a is the gas-liquid interface specific area (expressed in m⁻¹) and V₁ is the liquid volume in the reactor (expressed in m³).

[0025] The three curves reported in Fig. 5 refer to:

• system reactor + stirring device object of the present invention (Fig. 3), with rotation rate N = 5.8 sec⁻¹ and specific power W = 2.0 - 2.6 Kw/m³ (symbol ●);
• system reactor + stirring device of the prior art (Fig. 4), with rotation rate N = 10 sec⁻¹ and specific power W = 6.5 - 10 Kw/m³ (symbol ▲);
• system reactor + stirring device of the prior art (Fig. 4), with a rotation rate N = 6.5 sec⁻¹ and specific power W = 1.8 - 2.8 Kw/m³ (symbol ■).

[0026] The specific power, which is a quantity correlated to the shear stress, varies upon varying the filling level of the reactor.

[0027] From the comparison of the graphs reported in Fig. 5, it is clear that with the stirring device object of the present invention it is possible to obtain a substantially constant mass exchange between gas phase and liquid phase when the reactor filling level varies, while with the device of the prior art very large fluctuations are observed. It is also important to note how, the specific power being equal, the device object of the present invention assures a much higher mass exchange between the phases.

Claims

1. Rotating stirring device (1), particularly for mixing heterogeneous systems in mechanical stirred reactors, fit to be joined to a drive shaft coaxial with respect to the device rotation axis, characterized in that it comprises a plurality of extended and substantially C-shaped stirring elements (2), said stirring elements being symmetrically arranged with respect to the rotation axis and being joined each other at the respective ends correspondently with the rotation axis.

2. Device according to claim 1, in which the stirring elements (2) are concave blade-shaped with the concavity turned towards the device (1) rotation direction.

3. Device according to anyone of the previous claims, in which the stirring elements (2) are three.

4. Device according to anyone of the previous claims, in which each stirring element (2) consists of a lower end part (3), a vertical part (4) and an upper end part (5), such that the lower end part (3) is longer than the upper end part (5), while the vertical part (4) defines with the rotation axis an angle (B) comprised between 0° and 10°.

5. Device according to claim 4, in which the lower edge of the lower end part (3) forms with the horizontal plane an angle (C) comprised between 0° and 45°.

6. Reactor of substantially cylinder shape, characterized in that it contains a stirring device (1) according to claims from 1 to 5.

7. Reactor according to claim 6, characterized in that it has a diameter (T) and a height (V), while the stirring device (1) has such sizes that: the base diameter (D) of said stirring device (1) is comprised between 1/3 and 2/3 of the diameter (T); the bending radius (E) of the stirring elements (2) is higher than or equal to 1/3 of the diameter (T); the height (F) of the lower end part (3) is higher than or equal to 1/5 of the diameter (D); the width (G) of the vertical part (4) is comprised between 1/20 and 1/4 of the diameter (T); the height (L) of the stirring elements (2) is comprised between 3/2 of the diameter (D) and a value about equal to the height (V).

8. Reactor according to claim 7, in which the height (M) of the upper end part (5) of the stirring elements (2) of the device (1) is at least equal to 1/20 of the diameter (T).

9. (Co)polymerization process in emulsion or suspension, carried out in a reactor comprising the stirring device according to claims from 1 to 5.

10. Process according to claim 9, in which fluorinated olefinic monomers are (co)polymerized.

Drawing